Comments on: Multitasking in the AVR Microcontroller

By: Jeremy

Jeremy — Wed, 02 Apr 2008 15:45:50 +0000

Have you checked out the MakeController from Makingthings? It’s also an Atmel (ARM7 in this case), includes a crapload of I/O, and runs a true RTOS? Did I also mention it uses g++ and has a ROM bootloader?

I admit, it’s quite a step up from the AVR, but Makingthings has made it reasonably easy to use. For the project you’ve selected, your AVR should be more than enough; but it is quite easy to do SAM7X code on the Mac, and the MakeController is a great place to start.

By: Tim

Tim — Mon, 31 Mar 2008 03:57:35 +0000

I would stick with whatever is more readable. As long as you are just dealing with constant powers of two, compiler optimization should be able to convert the modulo operation into the bit-wise AND operation, so there should be no performance difference. I’m pretty sure this is true for the gcc versions I use for embedded ppc development; I’m not sure about this platform. Premature optimization is no more justified in the embedded space than it is anywhere else.

By: Wes

Wes — Mon, 17 Mar 2008 21:50:27 +0000

I’m gradually dragging my mindset into embedded programming and, as you can tell, I’m not entirely there yet.

Its certainly a different mindset and its great fun to be working at such a low level, especially when it gets to the point where every cycle counts.

By: bbum

bbum — Mon, 17 Mar 2008 17:09:03 +0000

Oh, duh! Of course!

Thanks, Aaron & Wes.

I’m gradually dragging my mindset into embedded programming and, as you can tell, I’m not entirely there yet.

By: Aaron

Aaron — Mon, 17 Mar 2008 14:36:35 +0000

I think his point was that “ledState = ledState & 3;” is equivalent to “ledState = ledState % 4;” and just requires a logical and op whereas modulo requires a division which will end up being done in software on something like the AVR as it doesn’t have a divide op. Using “ledState = (ledState==3)?0:(ledState+1);” would let you do something other than a power of 2 and is probably slower than the logical and.

Sure, you may have cycles to burn now, but just keep this in mind for when you don’t.

By: bbum

bbum — Mon, 17 Mar 2008 03:05:05 +0000

Nope — the modulus is there to bound the values between 0 and 3. Whether a conditional + assignment would be more efficient, I know not (haven’t looked at the assembly).

In this case, the CPU is running at 1mhz and I have cycles to burn.

[I am, of course, completely wrong. Wes's suggestion is dead on.]

By: Wes

Wes — Mon, 17 Mar 2008 03:01:00 +0000

ledState = ledState % 4;

Wouldn’t ledState &= 0b11 be vastly more efficient than a modulus operation?